High temperature ceramic composition



2,766,129 HIGH TEMPERATURE CERAMIC COMPOSITION No Drawing. AppiicationSeptember 21, 1953, Serial No. 381,500

Claims. (Cl. 106-48) This invention relates to a ceramic composition andmore particularly to a ceramic composition which when applied as acoating to stainless steel will protect the stainless steel fromoxidation at high temperatures.

The object of the present invention is to provide a ceramic coating forstainless steel which will protect the stainless steel fromintergranular oxidation at elevated temperatures in the range of 1800 to2000 F Intergranular oxidation of stainless steel frequently occurs whenthe stainless steel is subjected to high temperatures for extendedperiods of time. To prevent this oxidation, attempts have been made tocoat the stainless steel with various ceramic compositions. However, theceramic compositions have not proven satisfactory when applied tostainless steel and maintained at high temperatures for long periods forthey tend to crack, spall, or permit oxygen penetration and thesubsequent formation of oxides beneath the coating.

This invention is directed to a high temperature ceramic composition forapplication to stainless steel which is formed by mixing a base frithaving a high viscosity at elevated temperatures with refractoryadditions of alumina and lanthana. This mixture, having the necessaryadditions of clay and water to form a slip, is applied to the stainlesssteel as a thin coating. The coated stainless steel is fired at atemperature of about 1900 F. to fuse the ceramic composition to thestainless steel. The resulting coating will protect the stainless steelagainst oxidation at temperatures up to 2000 F. for periods of over 100hours.

The base frit of the ceramic coating 'has the following generalcomposition by weight:

Percent BeO 2.8- 3.1 K20 3.5- 4.1 MgO 1.3- 1.8 NazO 8.0-1l.0 A1203 5.5-7.0 SiOz 71.0-74.0 TiOz 2.5- 4.0

A preferred composition of the frit by weight is as The beryllia, BeO,has a high ionic potential and thereby functions to hold the ceramicstructure together.

The potassium oxide, K20, and the magnesium oxide, MgO, serve toincrease the viscosity of the material and maintain the composition in anon-fluid state at elevated temperatures.

2,766,129 if'atented O t- 9,

The soda, NazO, functions as a carrier or flux and gives body to thefrit.

The alumina in the frit, A1203, serves as a glass stabilizer and givesthe resulting ceramic composition a firm structure. The alumina preventsdevitrification of the ceramic.

The silica, SiOz, functions as a glass former and gives the compositiona high resistance to heat.

The titanium dioxide, TiOz, serves to improve the adherence of theceramic composition to the stainless steel. The slip general-1y has thefollowing composition in parts by weight:

Frit Alumina 18-22 Lanthana 8-10 Clay '8-10 Water 61-65 A preferredillustration of the composition of the slip The alumina and lanthanacombine to provide the ceramic composition with high refractoryproperties and an increased softening point. The alumina and lanthanaare preferably employed in a weight ratio of about two parts alumina toon part lanthana. This ratio may vary slightly depending on theparticular properties desired in the resulting ceramic but generally atwo to one ratio of these compounds produces a very satisfactorycomposition. The exact nature of the co-action between the alumina andlanthana is not known other than the elimination of one of the compoundsor a substantial change of the 2 to 1 ratio therebetween will result ina coating which will not satisfactorily adhere to stainless steel attemperatures in the neighborhood of 2000 F.

The clay employed in the slip may be any of the conventional enamelingclays or a mixture of the usual clays employed as a suspending agent inceramic compositions.

In addition a small amount of an electrolyte, such as sodium nitrite,NaNOz, or any other compound commonly employed as an electrolyte inceramic compositions, may be added to the slip to change the state ofaggregation of the clay and control the flow and suspensioncharacteristics of the same.

The amount of water employed in the slip helps determine the consistencyof the same. While a proportion of 61 to 65 parts by weight of water ispreferred for most coating applications, this proportion may be variedover wide limits depending on the consistency desired for any particularapplication.

The slip may be applied to any type of stainless steel which will notwarp on heating at a temperature of approximately 2000" F. or haveexcessive critical changes on cooling from that temperature.

Stainless steel of the austenitic variety, such as types 309 and 310,having 25% chromium-12% nickel and 25 chromium-20% nickel, respectively,are examples of stainless steels to which the composition of the presentinvention may be successfully applied.

The slip is applied as a thin coating to the stainless steel article.The coating may preferably have a thickness of .001 to .003 of an inchand this coating presents a dense surface having firm cohesive bondswhich prevent oxygen from penetrating through the coating to thestainless steel.

To prepare the base frit, the ingredients are mixed to gether inpowdered form. The mixture is then heated in a suitable furnace to atemperature of about 2550 F. until the mixture is completely in themolten state. The heating may take about an hour to render theingredients to this molten form.

The melt is then poured into water and quenched to make the frit. Thefrit is then dried by any conventional means, such as heating over aflame, until the water vapor is driven off.

The dried frit is then ground to a powdered state and mixed in theproper proportions with the alumina, lanthana, clay and water to formthe slip.

The stainless steel article is thereafter coated with the slip bydipping, spraying or the like, dried and subsequently fired at atemperature of about 1900" F. for a period of about minutes.

The bisque fuses to the stainless steel during the firing to provide adense hard coating which will effectively protect the stainless steelagainst intergranular oxidation at temperatures up to 2000 F. Theceramic composition of the invention has successfully protected thestainless steel against oxidation at high temperatures for extendedperiods of time of over 100 hours.

Various modes of carrying out the invention may be employed within thescope of the accompanying claims which particularly point out anddistinctly set forth the subject matter regarded as the invention.

I claim:

1. A base frit for a ceramic composition to be applied to stainlesssteel to protect the same against oxidation at high temperaturesconsisting by weight of about 3.0% BeO, about 3.8% K20, about 1.7% MgO,about 9.9% NazO, about 6.1% Al202, about 3.0% TiOz and about 72.5% Si02.

2. A ceramic composition to be applied to stainless steel as a coatingto protect the stainless steel from oxidation at elevated temperatures,consisting by weight of about 100 parts of a base frit, about parts ofalumina, about 10 parts of lanthana, about 9 parts of clay and about 63parts of water, said frit having the following composi- 1 tion byweight:

Percent BeO 3.0 K20 3.8 MgO 1.7 Na2O 9.9 A1203 6.1 TiO2 3.0 Si02 72.5

3. A base hit for a high temperature ceramic composition to be appliedto stainless steel to protect the same against oxidation at hightemperature consisting of by weight:

Percent BeO 2.8- 3.1 K20 3.5- 4.1 MgO 1.3- 1.8 Na20 8.0-11.0 A1202 5.5-7.0 Ti02 2.5- 4.0 SiO2 71.0-74.0

Percent BeO 2.8- 3.1 K20 3.5- 4.1 MgO 1.3- 1.8 NazO 8.0-11.0 A1203 5.5-7.0 Ti02 2.5- 4.0 Si02 71.0-74.0

5. A composite structure comprising an austenitic type stainless steelbase, and a ceramic composition applied to the base as a coating toprotect the stainless steel against oxidation at temperatures up to 2000B, said ceramic composition having the following composition in parts byweight:

'Base frit Alumina 18-22 Lanthana 8-12 Clay 8-10 Water 61-65 said frithaving the following composition in percentage by weight:

Percent BeO 2.8- 3.1 K20 3.5- 4.1 MgO 1.3- 1.8 NazO 8.0-11.0 A1202 5.5-7.0 Ti02 2.5- 4.0 SiO2 71.0-74.0

No references cited.

1. A BASE FRIT FOR A CERAMIC COMPOSITION TO BE APPLIED TO STAINLESSSTELL TO PROTECT THE SAME AGAINST OXIDATION AT HIGH TEMPERATURESCONSISTING BY WEIGHT OF ABOUT 3.0% BEO, ABOUT 3.8% K2O, ABOUT 1.7% MGO,ABOUT 9.9% NA2O, ABOUT 6.1% AL2O3, ABOUT 3.0% TIO2 AND ABOUT 72.5% SIO2.